This invention relates generally to drive circuits for display devices and particularly to a system for applying brightness signals to the pixels of a display device, such as a liquid crystal display.
Many display devices, such as liquid crystal displays, are composed of a matrix of pixels arranged horizontally in rows and vertically in columns. The data to be displayed are applied as brightness (gray scale) signals to data lines which are individually associated with each column of pixels. The row of pixels are sequentially scanned and the pixels within the activated row are charged to the various brightness levels in accordance with the levels of the brightness signals applied to the individual columns. In a color display each pixel is composed of at least three pixel elements which individually emit one of the primary colors of light red, green or blue. In an active matrix display each pixel element is associated with a switching device which is used to turn the individual pixel elements on and off. Typically, the switching device is a solid state device, such as a thin film transistor (TFT), which receives the brightness information from solid state circuitry. Because both the switching devices and the circuitry are composed of solid state devices it is preferable to simultaneously fabricate the switching devices and the circuitry utilizing either amorphous silicon or polysilicon technology. Liquid crystal displays are composed of a liquid crystal material which is sandwiched between two substrates. At least one, and typically both of the substrates, is transparent to light and the surfaces of the substrates which are adjacent to the liquid crystal material support patterns of transparent conductive electrodes arranged in a pattern to form the individual pixel elements. The goal of the industry is to fabricate the various control circuitry components on the substrates and around the perimeter of the display at the same time that the solid state switching elements are fabricated.
Amorphous silicon has been the preferable technology for fabricating liquid crystal displays because this material can be fabricated at low temperatures. Low fabrication temperature is important because it permits the use of standard, readily available and inexpensive substrate materials. However, heretofore it has been felt that amorphous silicon technology can not be used because it has low mobility and thus can not operate at the speeds necessary for producing television displays. For these reasons it has heretofore been felt that fabricating the control circuitry on the same substrates as the display matrix would require the use of polysilicon because of its much higher carrier mobility. However, the disadvantage of polysilicon is the need for fabrication at high temperatures which requires the use of special and expensive substrate materials.
For these reasons there is a need for a liquid crystal drive circuit for applying the brightness signals to the pixel elements of a display device which can be fabricated utilizing either amorphous silicon or polysilicon technology. The present invention fulfills this need.